Photovoltaic systems with electrical grid connection are formed by a set of photovoltaic panels (also called photovoltaic array or generator) and a conversion stage (also called inverter) that conditions the energy produced by the panels and injects thereof into the electrical grid. These are generally private facilities, which seek to maximize the economic benefit obtained from the sale of the energy produced to power companies. Therefore, cheap, reliable and high efficiency inverters are looked for.
Traditionally, in such facilities a transformer has been included between the inverter and the electrical grid, which provides galvanic isolation between the facility and the electrical grid. However, the fact that the transformer operates at low frequency (50/60 Hz) increases the size, weight and price of the conversion stage, while reducing its performance.
One option for eliminating the low frequency transformer, while maintaining galvanic isolation of the facility, is to use inside the converter a DC/DC conversion stage with a high frequency transformer. The use of a high frequency transformer allows reducing the size and weight of the conversion stage; however it increases the complexity thereof, while reducing its performance and reliability.
The removal of the transformer (both high and low frequency) allows obtaining an easier, cheaper and lighter conversion stage, while improving the performance thereof. Therefore, in recent years, the use of transformerless conversion structures has become very popular.
Certain photovoltaic facilities require the ground connection (earthing) of one of the terminals of the photovoltaic array. In some cases, this requirement has technological nature. This is the case of facilities in which certain photovoltaic panels with thin layer are used, wherein by grounding the negative a premature degradation of the panel is avoided, or facilities in which the flow of ground currents through the parasite capacity of the photovoltaic array is desired to be completely eliminated in order to improve the electromagnetic behavior of the conversion stage. In other cases, the need of grounding is determined by the current legislation, such as the NEC (National Electrical Code) of the United States.
Typically, photovoltaic systems with electrical grid connection are connected to a grid of the type T-N (in which the neutral point of the grid is ground connected). In this type of electrical grid, the use of a conversion stage with galvanic isolation (with a low or high frequency transformer) allows grounding one of the terminals of the photovoltaic array without arising any problems in the operation of the conversion stage. However, grounding the photovoltaic array in transformerless conversion stages based on conventional conversion structures, such as the H-bridge or those shown in the patent documents DE10221592A1, DE102004030912B3 and WO2008015298A1 is not possible.
In order to solve these problems raised when grounding the photovoltaic array in transformerless conversion stages, new topologies such as that proposed in DE 196 42 522 C1, have been developed. However, the current injected to the electrical grid through this topology is pulsed, which requires a large output filter in order to absorb the current harmonics.
This problem is solved in DE 197 32 218 C1. This structure is based on the connection of two DC/DC converters: Zeta and Cuk. The control of the semiconductors is carried out in such a way that during the positive half-cycle of the grid voltage the converter behaves like a Zeta and during the negative half-cycle as a Cuk. However, in this topology, as in DE 196 42 522 C1, by lacking boost DC/DC stage, the power fluctuation characteristic of one-phase systems causes a ripple in the voltage of the photovoltaic array at a frequency equal to twice the grid frequency, which results in the reduction of the energy obtained in the photovoltaic array by fluctuating the voltage around the maximum power point.
US 2004 0164557 A1 describes a simple topology that allows connecting the photovoltaic array negative to ground. The operation of this topology is based on obtaining a bipolar DC voltage (positive and negative with respect to the photovoltaic array negative), i.e. a DC bus with the midpoint connected to ground. From this bus, the use of a half bridge allows obtaining sinusoidal output voltages. The fact of using the voltage of the photovoltaic array as positive voltage on the DC bus makes that the structure can not be used with voltages of photovoltaic array lower than the maximum voltage of the electrical grid.
DE 10 2006 012 164 A1 describes a topology that allows connecting only one of the input terminal to ground, thus achieving at its output a bipolar DC voltage. However, all semiconductors used must withstand the maximum bus voltage, which must be at least twice the maximum voltage of the grid, which increases the switching losses and reduces the performance of the conversion stage.
US 2008 0266919 describes another topology that allows obtaining a bipolar DC voltage. In this case two cuk-type DC/DC converters are used. However, the first DC/DC converter must handle all the power of the system. In addition, the semiconductors used must withstand the bus voltage, which must be at least twice the maximum voltage of the grid, which increases the losses of the topology, decreasing its performance.
Another topology that allows obtaining a bipolar DC voltage is proposed in WO 2008 151587 A1. This topology uses two transistors controlled by the same control signal, three diodes and two magnetically coupled windings for obtaining a bipolar output voltage. The proposed operating mode makes that, all the time, the current drawn by the DC/DC stage, seen from the photovoltaic array, be a pulsating current, and therefore, the current in the coil W1 will be greater than that circulating in the event that the current flows continuously to the DC/DC stage, with the consequent increase of losses in semiconductors and coils.
The topologies proposed in US 2004 0164557 A1, DE 10 2006 012 164 A1, US 2008 0266919, and WO 2008 151587 A1 are based on obtaining a bipolar DC voltage (bus with the midpoint connected to ground). In order to obtain an AC voltage from this bipolar voltage DC/AC topologies, as the half-bridge or NPC half-bridge, can be used:                1. Half-bridge: This is a simple conversion topology that only consists of two switching elements (of transistor-type with antiparallel diode). However, its modes of operation only allow obtaining two levels of output voltage: Vbus/2 and -Vbus/2, being necessary to use a large inductance in order to filter the current harmonics produced. On the other hand, since the used semiconductors must be capable of supporting all bus voltage, this topology has large switching losses.        2. NPC Half-bridge. In order to improve the behavior of the half-bridge, a DC/AC NPC half-bridge can be used as a conversion structure. It is a structure formed by 6 switching elements (4 are of transistor-type with antiparallel diode and 2 diode-type). This topology allows obtaining three levels of output voltage: Vbus/2, 0 and -Vbus/2. This will reduce the current ripple, compared to that in a half bridge, but the complexity thereof is increased by using 6 switching elements.        
The power fluctuation characteristic of the one-phase systems causes a ripple in the bus voltage. In order to reduce this variation large capacities are used. Using a half-bridge or NPC half-bridge requires a bus voltage of at least twice the grid voltage, twice of that used in DC/AC structures as the H-bridge, which will increase the size of the required capacity compared to that used on an H-bridge. This increase in capacity increases the cost and considerably increases the volume of the conversion stage.